Metallic liners are employed in back-end-of-line (BEOL) metal interconnect structures to provide adhesion strength between a metal fill structure and a dielectric layer which embeds the metal fill structure. The metallic liner and the metal fill structure collectively constitute a current-carrying structure, which may be, for example, a metal line, a metal via, or an integrally formed combination thereof. For reliable operation of the current-carrying structure, high adhesion strength between the metallic liner and the metal fill structure is necessary. The high adhesion strength between the metallic liner and the metal fill structure provides protection against electromigration as well as mechanical stability of the BEOL metal interconnect structures.
In most cases, an anneal is needed to increase the grain size of the material of the metal fill structure formed directly on the metallic liner. When the adhesion strength is not sufficient, voids may be formed at the interface between the metallic liner and the metal fill structure upon annealing of the current-carrying structure. Such voids increase the resistance of the current-carrying structure, degrading electrical performance.
Further, surface diffusion along the boundary between the metallic liner and the metal fill structure is often the dominant factor in determining overall electromigration performance of the current-carrying structure. Thus, the adhesion strength between the metallic liner and the metal fill structure is critical in determining electromigration resistance of the current-carrying structure. Any void at the interface between the metallic liner and the metal fill structure exacerbates deterioration of electromigration performance by providing an initial void that grows with progression of electromigration with usage of the current carrying structure, i.e., passing of electrical current therethrough.
Referring to FIG. 1, a prior art metal interconnect structure comprises a patterned dielectric layer 110, a metallic nitride liner 120, a metal liner 130, a Cu seed layer 150, and an electroplated Cu structure 160. The patterned dielectric layer 110 comprises a line trough and a via cavity located underneath the line trough. The metallic nitride liner 120, the metal liner 130, the Cu seed layer 150, and the electroplated Cu structure 160 fill the line trough and the via cavity to form a prior art current-carrying structure (120, 130, 150, 160), which is an integrally formed line and via structure.
The metallic nitride liner 120 is typically formed by physical vapor deposition (PVD). Since PVD is a directional deposition method in which the deposited material moves from the direction of the sputtering target toward a substrate on which deposition occurs, the step coverage of the metallic nitride liner 120 is always less than 1.0, i.e., less material is deposited on sidewalls of a structure than on planar surfaces. Thus, more material is deposited on a bottom surface of the line trench than on sidewalls of the via cavity. One way to increase the side-wall coverage is a combination of deposition and directional sputter etching. In this case, a metallic material deposited on a recessed bottom surface may be resputtered off the bottom surface and redeposited onto the sidewalls surrounding the recessed bottom surface by a directional sputter etching. Further, more material is deposited on an upper portion of the sidewalls of the via cavity than a lower portion of the via cavity. While chemical vapor deposition (CVD) and atomic layer deposition (ALD) methods are known to provide enhanced step coverage than PVD, impurities are often found in these films, which will degrade the quality of the film as an adhesion layer and a diffusion barrier layer.
The metal liner 130 is also typically formed by PVD. The metal liner 130 is non-conformal due to the directional nature of the deposition process as discussed above. The accumulation of more material in the upper portion of the via cavity than in the lower portion of the via cavity creates an overhang at the top of the via cavity that blocks deposition of material at the bottom of the via cavity. Such an overhang makes subsequent filling of the via cavity with a conductive material difficult since the materials will seal off the top portion, which prevent further deposition inside of the via. Further, the metallic nitride liner 120 and the metal liner 130 have higher resistivity than the electroplated Cu structure 160. For this reason, the thickness of the metallic nitride liner 120 and the thickness of the metal liner 130 need to be kept as small as possible. Particularly, the scaling of semiconductor devices in front-end-of-line (FEOL) requires that metal interconnect structure be correspondingly scaled down.
Scaling down of the thicknesses of the metallic nitride liner 120 and the metal liner 130 may create a reliability problem. Specifically, when the planar thickness of the metallic nitride liner 120 and the metal liner 130 is about 10 nm or less, the coverage on the sidewall may not be uniform or contiguous. Thus, a thin metal liner region 133 may be formed, especially at a bottom portion of a sidewall of a via cavity, at which the thickness of the metal liner 130 is thinner than neighboring areas. In some cases, the material of the metal liner 130 may be absent in the thin metal liner region 133.
As the Cu seed layer 150 is deposited directly on the metal liner 130 by PVD, the thin metal liner region 133 provides less adhesion between the Cu seed layer 150 and the metal liner 130. This is because Cu tends to have weak adhesion to the metallic nitride liner 120 in the absence of a metal liner 130, or through a thin portion of the metal liner 130, within the thin metal liner region 133. The via cavity and the line trough are filled with electroplated Cu structure 160 to form the prior art current-carrying structure (120, 130, 150, 160).
Referring to FIG. 2, the thin metal liner region 133 of FIG. 1 is prone to formation of a cavity 137 due to the weak adhesion of the Cu seed layer 150 to a thinner portion of the metal liner 130 and to the metallic nitride liner 120. In one case, the cavity 137 may be formed during an anneal that is typically performed after the electroplating of the electroplated Cu structure 160 to increase the grain size of the electroplated Cu material. The weak adhesion between the Cu seed layer 150 and the metal liner at the thin metal liner region 133 facilitates movement of Cu material, which induces formation of the cavity.
Even if formation of a cavity is avoided during the anneal, usage of the prior art current-carrying structure (120, 130, 150, 160) by passing current therethrough may induce formation of a cavity 137 by electromigration of Cu material. The thin metal liner region 133 is vulnerable to electromigration and formation of the void 137 because of the weaker adhesion of the Cu material.
In view of the above, there exists a need to provide a metal interconnect structure providing sufficient adhesion strength between a metal fill structure and a patterned dielectric layer without requiring an excessive thickness.
Particularly, there exists a need for a metal interconnect structure including a metallic liner structure which provides sufficient adhesion strength over the entirety of the surface of the metallic liner structure without generating a region of weak adhesion strength.